Nitrogen fixation in soybeans as influenced by cultivar and Rhizobium strain differences

Abstract

The effect of soybean variety and Rhizobium strain, as well as the level of nitrogen fertilizer, were tested in a field experiment in Greece. Three nodulating varieties were used and one non-nodulating variety as control, with two Rhizobium strains, one commercially available and one isolated from the soil of the experimental site. Nitrogen fertilizer labeled with 15N enabled the nitrogen derived from the atmosphere (Ndfa), from the soil (Ndfs), and from the fertilizer (Ndff) to be estimated. Statistical analyses showed that the combined variety-strain effect was responsible for most of the variation observed in all parameters, either measured directly or calculated, while the nitrogen level had no effect. The locally isolated strain generally performed equally as well as the commercial one. With the highest yielding variety in particular, the nitrogen-fixing efficiency of soybean nodules, expressed as plant total nitrogen in excess of that in the non-nodulated control in relation to nodule dry weight, was even superior. Taking into account the low fertilizer recovery by plants, it is suggested that adapted cultivars properly inoculated can fix most of the nitrogen they need for high yields. This is of great economic importance for crops with high nitrogen-fixing efficiency, like the soybeans used in this work, which can fix more than 200 kg N/ha, as 15N dilution and nitrogen difference evaluations have confirmed. The local cultivars in this connection were superior to the United States cultivar, while the difference between cultivars in symbiotic performance were not due to the ability to nodulate, but rather to a much higher nodule-specific efficiency. The commercially available strain was found to produce generally more nodules and the locally isolated strain produced less but larger and more efficient nodules. Finally, the grains from the nodulated plants contained significantly smaller quantities of Fe, Ni, Cu, Zn, and especially Mo compared with non-nodulated plants, as determined by X-ray fluorescence techniques.